As filter media for air filters, there have been frequently used those produced by blending glass fibers with binders and processing into a paper. However, these filter media suffer from some problems, for example, self-dusting due to fine fibers contained therein and dusting caused by deterioration due to chemicals such as hydrofluoric acid. In recent years, therefore, porous PTFE membranes, which are clean materials and highly resistant to chemicals, have been employed as filter media for air filters in the fields of semiconductor industry, etc. A porous PTFE membrane can be produced by molding PTFE into sheet and then stretching the sheet to thereby make it porous (see, for example, JP-A-7-196831 and JP-A-6-816802 (PCT); the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Sole use of a porous PTFE membrane thus obtained lacks rigidity. Therefore, in many cases the porous PTFE membrane is used by bonding an air-permeable supporting member to one surface there. The filter medium thus reinforced by the air-permeable supporting member is pleated so as to give a continuous W-shaped structure (hereinafter referred to as "pleating") and employed as a filter medium unit.
On the other hand, there are proposed various methods for producing laminates of porous PTFE membranes (see, for example, JP-A-57-131236 and JP-A-3-179038). JP-A-9-504737 (PCT) proposes a filter medium using such a laminate.
Namely, JP-A-9-504737 (PCT) discloses a filter medium showing a high collection efficiency which comprises a laminate comprising five layers of porous PTFE membranes having the same pressure drop, and air-permeable supporting members located at both sides of the laminate. In this filter medium, the air-permeable supporting members are loosely pleated together with the porous PTFE membrane laminate but not bonded thereto.
In a filter medium having an air-permeable supporting member bonded to one surface of a porous PTFE membrane, however, some of pores of the porous PTFE membrane are clogged by bonding and thus the pressure drop largely increases. On the other hand, a filter medium having air-permeable supporting members located at both sides of a laminate does not suffer from any decrease in the pressure drop, since the supporting members are not bonded to the laminate. In this case, however, the laminate and the supporting members are not integrated and thus are poor in handling characteristics. In this case, furthermore, the supporting members are not bonded to the laminate and thus sometimes give deviations in the step of the pleating, thereby causing undesirable bends in the laminate or supporting members.